The present invention relates to a multi-carrier receiver.
Such a multi-carrier receiver is already known in the art, e.g. from the article ‘Mitigation of radio Interference in xDSL Transmission’ from the authors Luc de Clercq, Miguel Peeters, Sigurd Schelstraete and Thierry Pollet. This article has been published in IEEE Communications Magazine, March 2000, pag. 168-173. The DMT (Discrete Multi Tone) receiver drawn in FIG. 3 of this article and described on page 172 in the paragraph entitled ‘Digital RFI Canceling’ receives a multi-carrier DMT signal and demodulates digital data contained in DMT symbols from the set of carriers constituting the multi-carrier DMT signal. In order not to disturb radio amateur transmission, the PSD (Power Spectral Density) of the multi-carrier DMT signal stays below a certain maximum value within predetermined, standardised frequency bands: the so called RFI (Radio Frequency Interference) bands listed in table 1 of the cited article. A radio amateur signal or narrowband interferer transmitted within this RFI band can disturb receipt of the multi-carrier DMT signal, even outside the RFI band. Indeed, harmonics (sidelobes) of the radio amateur signal can affect the carriers of the DMT signal located outside the RFI band so that proper demodulation of the bits modulated on these carriers is no longer possible without additional measures. For this reason, the known DMT receiver estimates the characteristics of a narrowband disturber in the RFI band. These characteristics generally speaking are parameters of the RFI model that have to be estimated. If for the sake of simplicity it is supposed that the narrowband disturber is a sine or cosine shaped signal (the sine or cosine model in practice is not used but gives an indication why two parameters can be considered), the amplitude and phase are for instance two characteristics that can be estimated. To estimate these characteristics, the known DMT receiver selects antennas within the RFI band. An antenna is a reserved carrier out of the set of carriers that constitute the DMT signal, whereto no or a substantially low amount of power and data bits are assigned. All carriers within the RFI band are candidate antennas because the PSD (Power Spectral Density) of the multi-carrier DMT signal anyhow has to stay low in the RFI band, e.g. −80 dB/Hz for aerial wires (strong coupling PSD) and −60 dB/Hz for buried wires (weak coupling PSD) according to standard specifications. By sensing the signals received at the frequencies of the antennas, the multi-carrier receiver can determine characteristics of the narrowband disturber. This is explained at page 172 of the cited IEEE publication, right column, second paragraph.
The location of the antennas relative to the RFI signal frequency is critical for the estimation of the RFI signal characteristics. In case the narrowband disturber is located e.g. near an edge of the RFI band, the prior art DMT receiver cannot estimate the characteristics of the narrowband disturber accurately, because the two antennas cannot be located optimally. One of these antennas has to be selected too close to the RFI disturber, or both antennas have to be chosen at the same side of the RFI disturber.
A consequence of the operation of the known DMT receiver is loss of minimum guaranteed capacity. This minimum guaranteed capacity is the capacity (measured in bits per second received) in the worst case scenario. This worst case scenario corresponds to a situation where the narrowband disturber is located at an edge of the RFI band, because the harmonics of the narrowband disturber then affect the carriers out of the RFI band most strongly. In particular in this worst case situation, the prior art DMT receiver fails to accurately determine the characteristics of the narrowband disturber so that the disturbing effects thereof cannot be compensated for optimally.